1. Field of the Invention
This invention relates generally to static VAR generators that supply reactive power to rapidly fluctuating loads and, more particularly, to VAR generators that supply reactive power having control techniques that minimize the response time for effective compensation for rapidly varying electrical loads.
2. Description of the Prior Art
A static VAR generator is generally comprised of three .DELTA.-connected inductors controlled by anti-parallel thyristors that are excited every half cycle to provide a balanced reactive current component to an A.C. network to compensate for heavy reactive loads in the system. As shown in FIGS. 1A and 1B, typical for all three phases, the thyristors are fired at control angles .alpha. at points which relate to peak line-to-line voltages which correspond to the zero current crossover points to minimize instability in the electrical network.
U.S. Pat. No. 3,999,117 issued Dec. 21, 1976 to L. Gyugyi et al. teaches a control scheme for firing anti-parallel thyristors to insert reactors into an A.C. network for compensation thereof. This patent discloses how the three firing angles, .alpha..sub.12, .alpha..sub.23 and .alpha..sub.31, representative of the three phases of a three-phase system, are electronically computed. First, the required currents in the thyristor-controlled reactors are calculated by a desired inductor current computing circuit from the currents or power drawn by the individual phases of the load during half or full cycle intervals just preceding the earliest firing point .alpha..sub.c. Second, the firing angles are computed by a current-to-firing angle converter using an appropriate mathematical relationship between the firing angle and the fundamental component of current in the thyristor-controlled reactor. These computations, which take place in each half or full cycle, are synchronized to the A.C. supply voltages by a timing pulse generator. The firing angle for each reactor can be set once in each half cycle. This means that the output current in each phase of the VAR generator can be adjusted only once in each half cycle. The maximum range of the firing angle control, measured from the peak of the applied voltage, is 90 electrical degrees with .alpha..sub.c equal to zero; if .alpha..sub.c does not equal zero, then the range is 90.degree. minus .alpha..sub.c. The 90.degree. interval, in which the thyristor switch can be rendered conductive, may be termed the firing interval. The interval between the starting points of any two successive firing intervals may be termed computation interval, during which the desired inductor current is computed as shown in FIG. 1B.
In U.S. Pat. No. 4,068,159 issued to L. Gyugyi, et al. a static VAR generator is described that introduces a fixed delay angle .alpha..sub.c and reduces the inductance of the thyristor-controlled reactor so that .alpha..sub.c, the maximum required reactor current, is obtained. This method is beneficial in reducing the size of the reactor, and it may also improve the response time.
The most fundamental cause of time delay in state-of-the art static VAR generators is due to the fact that the current in the reactor can be changed only once in each half cycle. After the computations are complete, the output current for the VAR generator can be adjusted only once. Consequently, after the first sampling and computation time elapses, if the reacted demand should change suddenly, the VAR generator cannot further adjust its output until the next half cycle. It would be desirable for a VAR generator design to have the capability to allow further corrective action to be taken in response to any reactive demand change that may occur after the first sampling and computation time interval for each half cycle.